Electric lamp and filament therefor



June 5, 1934. s. RUBEN ELECTRIC LAMP AND FILAMENT THEREFOR Filed Nov. 29, 1929 l NVENTOR Pl/BEN Patented June 5,

UNITED STATES "PATE NT pmcs BLEc'rmoLAr/m AND THEREFOR Samuel Ruben, New York, N. Y., assignor to 'Sirian Lamp Company, Newark, N. J.,' a corpora ion of Delaware ApplicationNovembei- 29, 1929, Serial No. 410,310 (Cl. 176-131) I 5 type of electric lamp suitable foruse in house lighting, automobile lighting, projector illumination and various other types-of illuminating devices. The invention also pertains to a coated filament adapted tobeheated by the passage of electricity therethrough.

Among the objects of the invention is to provicle a filament adapted foruse in lamps and other means of light projection and illumination,

having a core or base of refractory material such as tungsten on which is placed a coating having such properties as to increase the light andlife efliciency of the filament.

Another object is to provide a coated filament in an electric lamp,'the coating being secured to the core substance by means which secure a minimum loss of heat efliciency while the filament is in use for illumination. 1

Still another object of the invention is to provide a coated filament havinga bonding agentcapable of resisting the high temperatures of an electric lamp filament to a degree approximately equivalent to that of the core and coating sub:

stances, V l 1 Another object'of the invention is the provision of a coated type filamentforan electric lamp wherein vaporization, especiallyin the presence of gas pressure, is noticeably reduced.

Additional objects of the invention reside in the improved apparatus and processes which re-.

11. Within the envelope 10 and mounted inthe base is the stem 12 which is extendedinto the filament su port 14. The .spider 15 is mounted on'the supportrod 14 and consists in a number of oppositely extending rods terminating in hooks 19. A similar spider 20 is mounted adjacent the base of the support rod above the stem at 21, the rods of this spider 20 also terminating in hooks 22. The filament 13 is wound over the spider hooks, as shown in Fig. 1, and the ends thereof secured to the electricterminals 23 and 24.-

In Fig. 2 is shown a cross-sectional view of the filament greatly enlarged to bring out the relative position of the layers or coatings which form the essence'of this invention. A filament of an elementary metal, such as tungsten or tantalum, is well known. These elements have been eminently satisfactory as filaments for electric lamps.

because of their refractory characteristics'and fairly low rate of vaporization, particularly when employed in conjunction with gas within the lamp envelope. .Theoretically, the tungsten filament, for example, may be improved by the utilization of a coating thereon which would have light radi ating properties more selective to visible radiations than tungsten alone. Certain materials, such as combinations of thorium and cerium, as used in the Welsbach mantle, are illustrative of this type of illumination. Another field of immovement in a tungsten lamp would be toward the utilization of a coating which would decrease the vapor pressure. Such improvements I have, in a measure, accomplished in the present invention, which utilizes a core material 16, made of tungsten or similar refractory metal such as tantalum on which I have provided an intermediate coating of beryllium aluminosilicate andan outer surface layer of beryllium oxide, these two, coatings being intimately associated with each other and bound together to form an integral filament.

In determining the constituent materials of the coatings, I found that for insulators the lower the, atomic number of the metal elements entering into the insulator the higher the specific resistance thereof and the higher the temperature magnesium, aluminum, silicon and beryllium have insulative characteristics at much higher temperatures, such temperatures varying substantially inversely in each group as the atomic numbers, beryllium and magnesium having the lowest numbers, and the compounds having the highest specific resistance, andthese maybe heated to extremely high temperatures without decomposing or losing their insulating qualities. Beryllium is better in this respect than magnesium, and has desirable properties as an illuminant, and for use with filaments of vacuum tubes therefore, where high temperatures and selective illumination are desirable, I prefer to use compounds of beryllium, although I do not'wish to limit myself to this particular metal.

To attain the objects of the invention I have found it necessary to provide a graded coating with a base layer next to the core wire which will fuse to the core at temperatures very much higher than normal operating temperatures, which will therefore act asa bonding agent between the core and an outer layer, with which outer layer it is adapted to have a slight chemical reaction so as to closely combine the two layers together. This outer layer must be highly refractory at the maximum operating temperatures.

The inner or base layer of the coating may be made by mixing finely ground beryllium aluminosilicate (3BeO,A12Oa6SiO2) with ammonium borate (NI-153204) in the proportion by weight of three of beryllium aluminosilicate to one of ammonium borate with enough water to give proper consistency. The beryllium aluminosilicate should be ground fine enough to fall through a size 250 screen mesh; that is, a screen mesh having 250 apertures to the inch. This mixture is coated upon the core wire which, as previously stated, may be of tungsten or any other desirable material having analogous or similar refractory and other properties, making it suitable for use in an electric lamp, and a current of about one and onehalf (1.5) amperes is then passed through the wire to heat it and dry the coating in place. I have found it preferable to apply two coats of this material to form the inner layer of the insulating coating, although more layers maybe added if found desirable. Two coats make the a thickness approximately 0.01 of a millimeter.

The outer layer is preferably made of beryllium oxide (B8203). The coating material for this oxide is prepared by grinding the oxide in a ball mill with twenty-five percent. by weight of ammonium borate, and enough distilled water to give the desired consistency. The mixture is then applied to the filament carrying the intermediate coating of beryllium silicate. This application is made while the filament is cold and the coated filament is then heated to about 500 C. at which temperature the ammonium borate decomposes, forming ammonia gas, water vapor and boric oxide. At least five successive coatings of beryllium oxide material should be applied. The successive heatings through the, coating process also cause a partial sintering of the coating material.

The wire thus provided with its insulating coating may be handled and bent and welded in position in a lamp or other appliance in which the wire is to be used, the coating having enough flexibility so that it will not crack or chip off.

If the wire is not to be used at once, preferably it should receive a collodion coating so as to avoid .moisture absorption by the coating. This coating may best be applied by placing the wire in a vessel containing a solution of collodion and allowing the wire to remain in this solution until the occluded gas escapes, being displaced by the collodion which then forms an adherent skin layer. Water absorption may also be avoided by subjecting the coating to a high temperature so as to vitrify the material thereof, and should be used preferably only when the filament is formed prior to heating.

When properly placed in a lamp, the lamp may be exhausted in the ordinary manner. To do this it is necessary to raise the temperature of the filament a great deal higher than it is heated in normal operation, the temperature in some instances reaching as high as 2,000 C. At this temperature the first or inner layer of beryllium aluminosilicate will fuse with the resistance wire and form a dense, hard, flexible coating which is substantially integral with the wire. The beryllium oxide, however, which forms the outer layer or surface of the insulating filament, will withstand temperatures in the neighborhood of 2400 C. without melting, and therefore is not affected by the high temperature used in exhausting the lamp. At the exhausting temperature, all physically or chemically unstable components of the coating are discharged so that the resistance wire and coating may be completely degasified. There is, however, a slight chemical action be tween the inner and outer layers which firmly knits the two together, thus making a substantially integral structure.

While beryllium oxide has highly desirable characteristics, as an external coating, and is used as a preferred material, I have found that aluminum oxide might be used with the beryllium oxide or even be used alone, although this material does not have all the desirable properties of beryllium oxide for lamp filament use.

As indicated above, still other materials may be used as an insulating coating, and I do not. therefore, desire to limit the invention to any specific material, except as defined in the claims hereto appended.

What I claim is:

1. In. a lamp construction the combination of an element, a base therefor, filament supporting means mounted within the envelope and on said base, and a filament mounted on the filament support, said filament consisting of a refractory base element, and a coating thereon formed of an insulating compound of a metal having low atomic number, and an outer coating of another insulating compound of a metal having low atomic number.

2. In a lamp construction the combination of an envelope, a base therefor, filament supporting means mounted within the envelope and on said base, and a filament mounted on the supporting means, said filament consisting of a refractory metal core, a layer of an insulating metallic silicate fused to the surface of the core, and an insulating oxide in close contact with said silicate.

3. In a lamp construction the combination of an envelope, a base therefor, filament supporting means mounted in the envelope and on said base, and a filament mounted on the support, said filament consisting of a refractory metal base, a coating of beryllium aluminosilicate fused to said base, and a coating of beryllium oxide closely associated with said fused coating.

4. In a lamp construction the combination of an envelope, a base therefor, filament supporting means mounted in the envelope and on said base, and a filament mounted on the support,said illament consisting of a refractory metal base, a coat ing of an insulating metallic silicate and a binder on said base, and an outer insulating coating of an oxide and a binder.

' 5. In a lamp construction the combination of an envelope, a base therefor, filament supporting means mounted in the envelope and on said and a filament mounted on the supportv said filament consisting of a refractory metal base, a coat ing of beryllium aluminosilicate mixed with the solution of ammonium borate on said base, and an outer coating of beryllium oxide mixed with the solution of ammonium borate on said first coating.

SAMUEL RUBEN. 

